An observational study of the three-dimensional structure of the wind field over Sacramento, California, is reported. The observations were made with a double-theodolite network during the summer period. Although the topography is relatively uniform, the analysis revealed a complex, three-dimensional structure in the wind field, including wind reversals with height and substantial horizontal variability. Spatial correlation functions showed that correlation increased with altitude and decreased with separation distance for the westerly and northerly components. It is suggested that the distance at which 50% of the variance is accounted for by the correlations might be an appropriate criterion for determining upper air grid spacing in air quality models. In the Sacramento data, this figure would be of the order of 10 km. Horizontal divergence and vertical velocities were calculated and revealed substantial subsidence over Sacramento during the study period. The diurnal variation of these quantities resembled that of the Los Angeles basin. The calculated subsidence was of the order of −5 cm s−1 and it is suggested that subsidence is a factor which should not be routinely ignored in air quality models.